The present invention relates to high speed telecommunications network switches and more particularly to a constant phase crossbar switch which compensates for phase inconsistencies in switched high speed data streams.
In a telecommunications device, such as a network switch, data units are received at input modules which are typically coupled to corresponding inputs of a switch matrix, otherwise referred to as a crossconnect, a crossbar or a crosspoint switch. The received data units are forwarded from the respective inputs of the crossbar switch to one or more output modules coupled to corresponding output modules of the crossbar switch. In a typical network switch, the input and output modules are disposed on printed circuit boards and the crossbar switch is likewise disposed on one or more circuit boards. The input/output modules and the crossbar switch are typically interconnected via a backplane.
It is known that the communication between the respective input and output modules and the crosspoint switch may be via parallel or links or serial links. Each approach poses its own unique problems in the design of a high speed network switch. It is well recognized that the use of parallel links between the input modules and the crossbar switch and the output module and the crossbar switch has the advantage of significantly reducing the data rate of the data stream through the switch thereby simplifying electrical design. For example to transfer one gigabit per second of data requires a one GHz clock if a serial data stream is used versus approximately a 31.25 MHz clock if the crossbar switch operates on a 32 bit wide parallel data stream. However, when the input and output modules are coupled to the crossbar switch using a wide parallel link or bus, a large number of interconnects must be accommodated. For example, in a 16 input port by 16 output port network switch, assuming single ended drivers and receivers and 32 bit wide parallel links per port, 512 interconnects would be required just for the parallel data links. If differential drivers and receivers are employed on the links, the number of interconnects would double. Such a design approach necessitates a large number of interconnects, has a potential effect on reliability of the system due tootle need to run large numbers of signal paths through connectors, and greatly complicates board layout and design.
The desire to achieve high aggregate data transfer rates in a small form factor thus favors the use of serial interconnects between the input modules and the crossbar switch and the crossbar switch and the output modules. The serial data stream approach minimizes the interconnect cost for a given system bandwidth and conversely, maximizes the system bandwidth for a given interconnect cost.
In a typical network switch employing a serial interconnect to a crossbar, a transmitter within the input module transmits a serial data stream over a link to an input of the crossbar switch and the crossbar transmits the serial data stream through one or more outputs to a receiver disposed on an output module. The transmitter comprises a high speed parallel to serial converter. Parallel data is strobed into the transmitter. The transmitter serializes the data and transmits the data as a high speed serial data stream to a corresponding input of the crossbar switch. Typically, one transmitter per port is coupled via a serial interconnect to a corresponding input of a N.times.N crossbar switch.
A crossbar switch has the characteristics of being able to effectively couple the data stream appearing at any specified one of the crossbar switch inputs to one or more of the crossbar switch outputs. The crossbar switch may not couple data streams appearing on any two inputs of the crossbar switch to any single output of the crossbar switch at any given time. One implementation of a crossbar switch known in the art comprises a set of cascaded multiplexers which are configured to allow any input to be connected to any one or more outputs of the crossbar switch subject to the above referenced constraint.
The receiver is essentially the inverse of the transmitter. The receiver receives the high speed serial data stream from the crossbar switch and deserializes the data to produce parallel data at its output.
At high serial transfer rates, it becomes impractical to recover the data at the receiver using a distributed clock source. The distribution of clock and data over separate interconnects undesirably adds to the number of interconnects. The minimum number of interconnects, and thus the highest density system, is achieved by encoding clock and data over a one bit wide serial interconnect. Since the serial data stream in such a circumstance carries both clock and data, the receiver must regenerate the clock in order to recover the transmitted data. Phase locked loops (PLLs) have been commonly employed in the receiver to accomplish this function.
In a system employing high speed serial transmission through a crossbar switch at one point in time a first transmitter may be sourcing a serial data stream to a particular receiver and following a switching event at the crossbar switch, a second transmitter may be sourcing a different high speed serial data stream to the same receiver. The serial data streams may be clocked at data rates of over one gigabit per second. At such data rates, phase discontinuities between the respective data streams appear at the inputs of the respective receivers at crossbar switching events due to variations in the printed circuit path lengths for the various circuits, different propagation delays as well as other reasons. It is impractical to tune the delays during the design process such that the delay time from a transmitter through the crossbar switch is constant or predictable. As a result, when the crossbar switch changes the transmitter that a given receiver is connected to, a phase discontinuity in the serial bit data stream is often seen by the receiver. The phase discontinuity causes the receiver to lose synchronization of the bit stream. After some period of time, the phase locked loop associated with the respective receiver will reacquire lock on the new data stream such that it can reliably reclock and recover the data. The extended time period to reacquire lock lowers the effective bandwidth of the system since data cannot be transmitted during this interval.
More specifically, in one high-speed telecommunications network switch implemented by the present assignee, a one gigabit serial data stream comprising a preamble, a sync field, a data field and a postamble is switched through a crossbar switch. The preamble comprises 60 alternating ones and zeros, the sync field comprises a predetermined 10 bit code defining the start of the data field, the data field contains 560 data bits and the postamble contains 10 bits which are employed in conjunction with an 8B/10B encoding technique to maintain DC balance of the transmitted data stream. The receiver must thus reacquire lock on the data stream within the period afforded by the 60 bit preamble if lock is lost following a crossbar switching event in which a different transmitter is coupled to a particular receiver.
It is extremely difficult to construct a phase locked loop which can reacquire lock within 60 bit times at 1 gigabit per second or higher data rates. Various alternatives are routinely considered. First, the percentage of the specified cycle for clock synchronization may be increased; i.e. the length of the preamble may be increased and the length of the data field may be correspondingly decreased. This approach allows a greater number of bits for the receiver to achieve lock, however, it undesirably reduces the percentage of the available cycle that is available to carry the payload. Alternatively, the length of the preamble may be increased to provide a sufficient length bit stream for the receiver to reacquire lock after a switching event and, at the same time, the number of bits in the payload may be increased such that the percentage of the preamble bit length to the overall bit length for the cycle has not changed. This approach results in a reduction in the frequency of switching events through the crossbar switch which renders the switch less efficient in handling incoming traffic.
Another approach which has been utilized involves the duplication of receivers and transmitters on both sides of the crossbar switch. More specifically, a transmitter is employed to encode and serialize the data stream and to forward the serial data over an interconnect on the backplane for receipt by the crossbar switch. A receiver having a phase locked loop is located prior to the input to the crossbar switch and is employed to recover the clock and to deserialize the data. The parallel data is then switched through the crossbar switch at a reduced clock rate. At each output of the crossbar switch, the parallel data is coupled to a transmitter which encodes the data for serial transmission over a link for receipt by a receiver located on an output module. A phase locked loop associated with the receiver is employed to recover the clock and data transmitted by the respective transmitter at the output of the crossbar switch. This approach avoids the problems associated with phase discontinuities at switching events at the expense of an increased number of transmitters and receivers and a more complex crossbar switch.
It would therefore be desirable to be able to employ a high speed serial technique for transmission of data through a crossbar switch while avoiding the problems associated with phase discontinuities at switching events.